This research will examine how "molecular" (small-scale) variables affect choice behavior. One line of research will examine the effects of delay in situations where pigeons must choose between a small but immediate work requirement and larger but more delayed work requirement. An adjusting-delay procedure will be used, in which the delay for one alternative is systematically increased and decreased over trials to find an indifference point--a delay at which the two alternatives are equally preferred. A second line of research will examine choice in more complex procedures called concurrent-chain schedules, which feature a choice period followed by a period in which the subject receives the consequences of it choices. A third line of research will examine choice behavior during periods of transition-periods in which the subject must adapt to a change in the reinforcement contingencies. The subjects in most of the experiments will be pigeons performing in standard operant conditioning chambers with food as the reinforcer. However, because pigeons have been the subjects in most previous experiments on concurrent-chain schedules, a fourth line of research will use rats to test the generality of the findings obtained with pigeons. The results will be used to test and compare the predictions of several different mathematical models, such as a recently developed model called the "hyperbolic value-added model," which is designed to predict behavior in a wide variety of choice situations. Many important everyday choices involve a conflict between an individual's short-term and long-term interests (e.g., the pleasures of smoking, drinking, or overeating versus future health). The research described in this proposal is directed at developing a quantitative model for understanding how such choices are made, why individuals sometimes do and sometimes do not make choices that are in their best long-term interests, and what factors control these decisions.